Pretreatment process for a surface texturing process

ABSTRACT

Before submitting a sample, including a first material layered upon a substrate, to an ion milling process, whereby a second material is sputtered onto the surface of the first material and the sample is then submitted to an etching process, an irregularity is formed on the surface of the first material. The overall process results in the formation of cones, or micro-tip structures, which may then be layered with a layer of low work function material, such as amorphous diamond. The irregularity in the surface of the first material may be formed by polishing, sandblasting, photolithography, or mechanical means such as scratching.

This is a division of application Ser. No. 08/427,462 filed Apr. 24,1995 now U.S. Pat. No. 5,628,659.

TECHNICAL FIELD OF THE INVENTION

The present invention relates in general to a surface texturing process,and more particularly, to a pretreatment process performed prior toperforming a surface texturing process wherein irregularities areproduced on a surface to be textured using an ion beam.

BACKGROUND OF THE INVENTION

Surface texturing has many applications in electron sources, electricbonding, solar cells, optics and display devices. One of the mostcommonly used technologies for surface texturing is ion beam sputtering.The details of this process are described in Cone Formation as a Resultof Whisker Growth on Ion Bombarded Metal Surfaces, G. K. Wehner, J. Vac.Sci. Technol. A3(4), pp. 1821-1834 (1985) and Cone Formation on MetalTargets During Sputtering, G. K. Wehner, J. Appl. Phys., Vol. 42, No. 3,pp. 1145-1149 (Mar. 1, 1971), which are hereby incorporated by referenceherein, which teach that a cone (micro-tip) structure may be produced byusing an ion source for etching a material, such as copper, after it hasbeen seeded with a second material, such as molybdenum. An improvementupon this process is the subject of the above cross-referenced patentapplication, wherein a dual ion beam system is utilized to efficientlycontrol the ion beam sputtering process.

For many applications, it is required that the surface texturedstructure be uniform in terms of the density, height and shape of theresultant cones. Additionally, for mass production manufacturing, theprocess should be controllable and repeatable. However, the above-notedsurface texturing process using ion beam sources is difficult tocontrol, and the structures of the cones are not as uniform as manyapplications require, such as for flat panel display devices.

Thus, there is a need in the art for a surface texturing process that iscontrollable, repeatable, and produces a uniformity of resultant cones.

SUMMARY OF THE INVENTION

The aforementioned need is satisfied by the present invention, whichforms micro-tip structures (throughout this document “micro-tip” and“cone” are used interchangeably) at or near the locations upon which anirregularity has been formed in the surface of a material.

Thus, when forming micro-tips, a material layer is deposited upon asubstrate. An irregularity is then formed in the surface of the materialprior to loading the sample into an evacuated chamber to be subject toan ion milling process. The ion milling process sputters a seed materialonto the surface of the material. An ion beam is then utilized to etchthe material, resulting in cones or micro-tips being formed thereon. Theirregularities are nuclei sites for collecting the seed material.

An irregularity may be formed by polishing the surface with a polishingsolution, such as a diamond spray.

Alternatively, an irregularity may be formed by bombarding the surfacethrough a sandblasting technique.

Alternatively, an irregularity may be formed using a photolithographytechnique, whereby a step feature is caused to be formed on the surfaceof the material.

Alternatively, an irregularity may be formed through a mechanical means,such as by scratching the surface with a needle.

In a preferred embodiment of the present invention, a dual ion beamsystem is utilized whereby one ion beam source is used to sputter theseed material onto the surface of the material, while a second ion beamsource is utilized for the etching portion of the process.

In one embodiment of the present invention, the material upon which theirregularities are formed is an emitter material that may be utilized ina field emission device, wherein electrons are emitted from the formedmicro-tips under the influence of a potential field.

The foregoing has outlined rather broadly the features and technicaladvantages of the present invention in order that the detaileddescription of the invention that follows may be better understood.Additional features and advantages of the invention will be describedhereinafter which form the subject of the claims of the invention.

BRIEF DESCRIPTION OF THE DRAWING

For a more complete understanding of the present invention, and theadvantages thereof, reference is now made to the following descriptionstaken in conjunction with the accompanying drawings, in which:

FIG. 1 illustrates a dual ion beam system for producing micro-tipstructures;

FIGS. 2 and 3 illustrate a formation of micro-tips;

FIG. 4 illustrates a preferred embodiment of the present invention;

FIG. 5 illustrates an alternative embodiment of the present inventionutilizing a polishing technique;

FIG. 6 illustrates an alternative embodiment of the present inventionutilizing a sandblasting technique;

FIGS. 7A-7B illustrate an alternative embodiment of the presentinvention utilizing a photolithography technique;

FIG. 8 illustrates an alternative embodiment of the present inventionusing mechanical means for creating defects in the surface of a sample;

FIG. 9 illustrates a sample having had dints produced thereon; and

FIG. 10 illustrates a top view of an apparatus fabricated in accordancewith the present invention.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION

In the following description, numerous specific details are set forth toprovide a thorough understanding of the present invention. However, itwill be obvious to those skilled in the art that the present inventionmay be practiced without such specific details. In other instances,well-known circuits have been shown in block diagram form in order notto obscure the present invention in unnecessary detail. For the mostpart, details concerning timing considerations and the like have beenomitted inasmuch as such details are not necessary to obtain a completeunderstanding of the present invention and are within the skills ofpersons of ordinary skill in the relevant art.

Refer now to the drawings wherein depicted elements are not necessarilyshown to scale and wherein like or similar elements are designated bythe same reference numeral through the several views.

Referring first to FIG. 1, there is illustrated dual ion beam system 10in accordance with the cross-referenced patent application Ser. No.08/427,462 now U.S. Pat. No. 5,628,659. The ion beams produced byKaufman ion source 13 are utilized to etch material 304, while Kaufmanion source 12 is utilized to sputter seed material onto material 304.Evacuated chamber 15 (alternatively chamber 15 may be filled with aparticular gas) may be utilized to enclose system 10.

Referring to FIGS. 1, 2 and 3, glass substrate 308 is first cleaned.Next, depending upon the particular structure desired, a layer of 700angstroms of chromium (Cr) is optionally deposited upon glass substrate308. Next, layer 305 is deposited using electron beam evaporation,sputtering or a CVD (chemical vapor deposition) process. Layer 305 maybe a resistive layer of 5,000 angstroms (0.5 μm) of amorphous silicon(a-Si). Thereafter, a 3 μm (micrometer) copper (Cu) film layer 304 isdeposited upon layer 305, preferably utilizing electron beamevaporation. This entire structure, which may eventually comprise thecathode of a flat panel display is then loaded into system 10 andcoupled to heater 11. Since the formation of the cones, or micro-tips,is a temperature-dependent process, heater 11 is used to assist incontrolling the entire process.

Ion source 13 is utilized to etch away portions of material 304, whileion source 12 is utilized to sputter a seed material, which ispreferably molybdenum (Mo), onto material 304. Ion source 13 ispreferably operated with a beam energy of 800 volts and a beam currentof 80 milliamps, while ion source 12 is preferably operated with a beamenergy of 800 volts and a beam current of 50 milliamps. The molybdenumseed material is sputtered onto material 304 by the bombardment ofmolybdenum target 14 with an ion beam from ion source 12.

The result of this process implemented within dual ion beam system 10 isthat portions of material 304 are etched away, resulting in cones, ormicro-tips, as illustrated in FIG. 3. Please refer to Cone Formation asa Result of Whisker Growth on Ion Bombarded Metal Surfaces and ConeFormation on Metal Targets During Sputtenng, which are referenced above,which teach that such a cone structure may be produced by using one ionsource for etching the material after it has been seeded with amaterial, such as molybdenum.

Ion beam source 13 etches away material 304 while ion beam source 12sputters a seed material from target 14 to deposit on the surface ofmaterial 304. Note that source 12 and target 14 can be replaced withother deposition equipment, such as RF sputtering.

The structure, density and height of tips 304 are sensitive to the ratioof the etching rate and the deposition rate of the seed material. Atoptimized conditions, the etching rate for Cu is 8 angstroms per secondand the deposition rate for Mo is 0.2 angstroms per second. Theseconditions are achieved at the above noted 800 volts beam voltage and 50milliamp beam current for source 12, and 80 milliamp beam current forsource 13. Very small amounts of seed material can give rise to seedcone formation in material 304. In the case of Mo seed atoms on Cu, forproducing cones, the ratio of Mo atoms arriving at material 304 can beas low as one seed atom per 500 sputtered Cu target atoms. In otherwords, the ratio of the deposition rate to the etching rate can be aslow as 1/500.

Control of this process is implemented with the assistance of massspectrometer 16, which is utilized to monitor the etching process. Oncemass spectrometer 16 detects a preselected amount of material 305, theetching process may be terminated. For example, if material 305 isamorphous silicon, then mass spectrometer 16 will monitor for apreselected amount of silicon. If a preselected amount of silicon ismonitored, then the process may be terminated either manually orautomatically. Please refer to U.S. patent application Ser. No.08/320,626 now abandoned, assigned to a common assignee, which is herebyincorporated by reference herein, for a further discussion of such aprocess.

Note that material 304 may also be comprised of Titanium (Ti) or silver(Ag), while molybdenum may be replaced by tungsten (W).

To improve the uniformity of the surface texturing, irregularities inthe surface of layer 304 may be produced. It has been found that cones304 such as those illustrated in FIG. 3 are more likely to grow in anarea where the surface of layer 304 has been “damaged” in some way. Thefollowing describes various alternative methods of “damaging” thesurface of layer 304.

Hereinafter, the sample comprising glass substrate 308 with layer 304will also be referred to as sample 53. Note that layer 305 is optional.

Referring to FIG. 4, there is illustrated a process for polishing thesurface of layer 304 with a LECO® Aerosol Diamond Spray beforesubmitting sample 53 for ion bombardment within system 10. The diamondparticles within the spray are 0.25 μm in diameter. The process beginsat step 401 and proceeds to step 402, wherein a fresh TX172 TEXWIPE®swab is saturated when the LECO® 0.25 μm aerosol diamond spray. Next, instep 403, sample 53 is laid face up on a clean lint-free tissue andswiped lightly from left to right approximately 46 times with thesaturated swab. The fluid is then allowed to evaporate from the surfaceof layer 304. Thereafter, in step 404, a fresh TX172 TEXWIPE® swab issaturated with isopropyl alcohol (“IPA”), which is utilized to swipe thesample from left to right approximately 25 times. In step 405, thesurface of layer 304 is then rinsed with IPA twice. In step 406, thesurface is dried with dry N₂. Next, in step 407, sample 53 is thenloaded into dual ion bean system 10 for the ion beam sputtering processdescribed above. Using this process, the uniformity of the conestructures (including height, shape and density) is improved. Thisprocess also results in an ability to produce the cones in a morecontrollable and repeatable manner.

Alternatively, the diamond spray solution may be replaced with adifferent polishing solution comprised of 50 nm Al2O3+DI-H2O (deionizedwater).

Referring next to FIG. 5, there is illustrated an apparatus forperforming an alternative embodiment of the present invention, which isideally suited for a large sample 53 size. Sample 53 is mounted tosample holder 52, which is a portion of polishing machine 50, which maybe a Model 372 polisher made by Westech Systems, Inc. As noted by thearrows in FIG. 5, polishing machine 50 has an ability to slide androtate sample 53 across the surface of polishing pad 54, which is aRodel 750 finishing pad, which is mounted upon flat surface 55. Sample53 may be rotated at 50 rpm. The contact pressure between pad 54 andsample 53 is preferably 1 psi (pound per square inch). The sample ispreferably polished for 30 seconds. During the polishing process, apolishing may be contained within container 51 is poured onto thesurface of polishing pad 54. This polishing slurry is 50 nm Alumina R90slurry (Al2O3:DI-H20=1:10 ratio).

The result of this polishing process is similar to the processillustrated with respect to FIG. 4, wherein defects are produced in thesurface of layer 304 (see FIG. 9).

Referring to FIG. 6, there is illustrated another alternative embodimentof the present invention, which utilizes a sandblasting process tocreate the defects on the surface of layer 304 before subjecting sample53 to the process of system 10. Sandblaster 60 (Trinco Dry Blast, Model48/EF2) is utilized to blast the surface of layer 304 with sandparticles 61 at approximately an air pressure of 15 psi. 90% ofparticles 61 are in the range of 148 μm-188 μm in diameter. The distancebetween nozzle 60 and sample 53 is preferably 15 inches. Surfacefinishing time is approximately one minute.

Referring next to FIGS. 7A and 7B, there is illustrated anotheralternative embodiment of the present invention, whereinphotolithography techniques are utilized to create a minute “step” onthe surface of layer 304. The cones are then grown along the edges ofthese created steps. With such a process, there is an ability toselectively grow the cones or micro-tips exactly where they are desired.

The first portion of this process is a deposition of a 700 angstromlayer of chromium 700 or an equivalent material on the clean surface ofglass substrate 308, utilizing a sputtering technology along withphotolithography in order to pattern film layer 700 into 50 μm widestrips having a separation therebetween of 50 μm. Of course, other widthstrips and separations may be utilized. Thereafter, a 3 μm copper film304 or an equivalent material is deposited on top of glass substrate 308with chromium strips 700 utilizing a sputtering technique. The result ofthis process is “steps” 701 and 702 on the surface of layer 304. Theedges of the “steps” 701, 702 are the imperfections upon which the conesare to be grown. The entire sample 70 is then mounted in system 10 forthe dual ion beam sputtering surface texturing process, which producescones 304 as illustrated in FIG. 7B.

Referring to FIG. 8, there is shown yet another alternative embodimentof the present invention wherein defects on the surface of layer 304 areproduced by mechanical means such as by scratching the surface of layer304 using a fine needle 80. Cones are then grown along the edges of thescratches.

As briefly stated above, the present invention results in cones grownnear or at defects or irregularities on the surface of material 304. Theaforementioned polishing processes create defects on the surface bycreating a number of small dints 90 as illustrated in FIG. 9. Thesurface of material 304 previous to producing any type of irregularityto any of the aforementioned techniques is quite smooth and damage-freeas a result of the deposition of layer 304 by RF sputtering. Theirregularities are the nuclei sites for cone growth because the seedmaterial (in this case molybdenum sputtered onto surface 304 by system10) is likely trapped inside or adjacent to the irregularities. In otherwords, pretreatments (e.g., polishing, steps, sandblast, etc.) createmany preferable sites for seed atoms to form clusters, which are thenuclei sites for cone growth.

Although the present invention and its advantages have been described indetail, it should be understood that various changes, substitutions andalterations can be made herein without departing from the spirit andscope of the invention as defined by the appended claims.

What is claimed is:
 1. A system for surface texturing a first material,said system comprising: means for depositing said first material on asubstrate; means for producing irregularities across an area of asurface of said deposited first material to increase uniformity of sitesfor forming structures; means for sputtering a second material onto asurface of said first material, wherein said substrate includes a layerof a third material on which said first material has been deposited;means for etching said first material; and a mass spectrometer formonitoring an electromagnetic spectrum originated at a location of saidfirst material for detecting a predetermined amount of said thirdmaterial.
 2. The system as recited in claim 1, wherein said sputteringmeans includes a first ion beam originating from a first ion beamsource, said first ion beam bombarding a target of said second material,and wherein said etching means includes a second ion beam originatingfrom a second ion beam source.
 3. The system as recited in claim 1,further comprising: means for growing microtips at the surfaceirregularities.
 4. A system for surface texturing a first material, saidsystem comprising: means for depositing said first material on asubstrate; means for producing irregularities across an area of asurface of said deposited first material to increase uniformity of sitesfor forming structures, wherein said means for producing irregularitiesacross the area of the surface of said deposited first material furthercomprises a means for polishing said surface with a polishing solution;means for sputtering a second material onto a surface of said firstmaterial; and means for etching said first material.
 5. The system asrecited in claim 4, wherein said polishing means further comprises apolishing machine coupled to said substrate, and a polishing pad onwhich said surface is polished.
 6. The system as recited in claim 4,further comprising: means for growing microtips at the surfaceirregularities.
 7. A system for surface texturing a first material, saidsystem comprising: means for depositing said first material on asubstrate; means for producing irregularities across an area of asurface of said deposited first material to increase uniformity of sitesfor forming structures, wherein said means for producing irregularitiesacross the area of the surface of said deposited first material furthercomprises a means for sandblasting said surface; means for sputtering asecond material onto a surface of said first material; and means foretching said first material.
 8. The system as recited in claim 7,further comprising: means for growing micro tips at the surfaceirregularities.
 9. A system for surface texturing a first material, saidsystem comprising: means for depositing said first material on asubstrate; means for producing irregularities across an area of asurface of said deposited first material to increase uniformity of sitesfor forming structures, wherein said means for producing irregularitiesacross the area of the surface of said deposited first material furthercomprises means for depositing a strip of a third material on saidsubstrate prior to depositing said first material on said substrateresulting in a production of a stepped surface on said first material;means for sputtering a second material onto a surface of said firstmaterial; means for etching said first material.
 10. The system asrecited in claim 9, further comprising: means for growing microtips atthe surface irregularities.
 11. A system for surface texturing a firstmaterial, said system comprising: means for depositing said firstmaterial on a substrate; means for producing irregularities across anarea of a surface of said deposited first material to increaseuniformity of sites for forming structures, wherein said means forproducing irregularities across the area of the said deposited firstmaterial further comprises a means for scratching said surface; meansfor sputtering a second material onto a surface of said first material;and means for etching said first material.
 12. The system as recited inclaim 11, further comprising: means for growing microtips at the surfaceirregularities.